The involvement of highly reiterated DNA (hrDNA) sequences in chromosomal evolution will be studied utilizing the unique resources of the San Diego Zoo. Closely related species with drastically different karyotypes (e.g. Callicebus torquatus, 2n equals 20, vs. Callicebus moloch, 2n equals 46, and Muntiacus muntjac, 2n equals 7 male, 6 female, vs. Muntiacus reevesi, 2n equals 46) will be employed in studies of the DNA sequence relatedness and cytological localization of hrDNA sequences between these species pairs. Of particular interest is the relationship between centromeric heterochromatin of the species with the higher diploid chromosome number to centromeric and interstitial heterochromatin of the related species with a reduced diploid chromosome number. In addition a program of comparative molecular cytogenetic studies will be initiated in an attempt to identify other useful model systems for molecular cytogenetic analysis. Cell lines derived from animals within the Zoo's collections and DNA's extracted from tissue samples obtained at autopsy will be used for these comparative studies. Sequence relatedness of hrDNA fractions (including density satellite DNA's) will be determined by reassociation kinetics and thermal stability of molecules containing hrDNA sequences. Cytological localization of hrDNA sequences will be accomplished by in situ nucleic acid hybridization of He-labeled RNA molecules prepared from hrDNA template molecules and bacterial RNA polymerase in conjunction with C-banding analysis.